This invention relates to zinc plating or electro-depositing, and more particularly to novel, non-cyanide zinc plating or electro-depositing baths of the alkali-zincate type and to the electro-deposition or plating of bright or semi-bright zinc therefrom. Additionally, this invention is also directed to a process for zinc plating utilizing the novel baths of the present invention.
Conventional zinc electro-depositing or plating baths and processes are generally classified as either a strong alkali type utilizing cyanide compounds as a primary constituent or acid type baths and processes using zinc chloride or zinc sulfate as a primary constituent. Of these, the strong alkali, cyanide type bath and process is most widely utilized in the industry.
While it is generally known that such strong alkali, cyanide-type zinc plating baths and processes can provide a zinc plated surface on a ferrous metal substrate which has sufficient smoothness and a semi-gloss appearance, large amounts of cyanide compounds are utilized in these types of strong alkali zinc plating systems. In such cyanide-type systems it is also conventionally known that small amounts of additional brighteners can be utilized to obtain an even brighter zinc plating. Such conventional brighteners include gelatin, peptone, sodium sulfide, thiourea, polyvinyl alcohol, aldehydes, ketones or salts of organic acids, either added singly or together with the other components of the conventional cyanide-type plating bath.
However, because the cyanide containing strong alkali-type zinc plating baths and processes utilize a large amount of cyanic compounds which themselves are toxic materials, they are becoming less acceptable because of their adverse effect on the environment. It is becoming impossible to discharge the waste solution as such. Accordingly, the cyanide containing alkali-type zinc plating baths and processes have substantial disadvantages, such as the necessity for extensive waste treatment facilities, the requirement for utilization of additional chemical agents for the treatment of the waste solution to remove the residual cyanides, unfavorable operating conditions and the risk of environmental pollution. Thus, from the aspects of operating efficiency, economy, and public safety the use of a zinc plating bath or process utilizing cyanic compounds is rapidly becoming unacceptable.
Furthermore, since zinc electroplating is applied directly onto a ferrous metal substrate in the majority of cases, iron is dissolved in substantial quantities in zinc plating baths containing cyanic compounds. In particular, a ferro ferricyanide complex salt containing cyanide and iron can form with a high degree of stability, so that it cannot easily be decomposed to recover the cyanide. However, in accordance with the present invention, it becomes possible to completely decompose the iron complex salt which forms in the zinc plating bath by use of conventional treatment techniques.
Accordingly, there has been a need in the industry for a cyanide-free zinc plating bath and process of the strong alkali type. Typical of the baths and processes developed to date is the alkali-zincate type bath containing sodium zincate and an excessive amount of sodium hydroxide. Unfortunately, when zinc plating is produced from this type of basic alkali-zincate bath, the zinc plating deposited on the substrate is lacking in brightness, smoothness of grain and adherence of the zinc coating. In order to improve the zinc plating provided by the conventional alkali-zincate system, attempts have been made to add brightening agents to the plating baths.
For example, it is known that suitable brighteners and additives for such an alkali-zincate zinc plating system are salts of gluconic acid, alkyl amines, and alkylene amines such as ethylenediamine, triethylenetetramine and tetraethylenepentamine. Furthermore, these brighteners and additives have been utilized alone or in combination with aromatic aldehydes. However, even with use of these conventional brightening agents, it is difficult to produce a uniform and homogeneous zinc plating on the substrate to be plated. For example, when aromatic aldehydes are utilized, care must be taken not to operate at higher temperatures, which would otherwise be desirable, since the aromatic aldehydes are not stable in alkali solution and their breakdown will be accelerated by the higher temperatures. Accordingly, since the operating and plating conditions utilizing the conventional types of brightening agents are limited, if good gloss, brightness and appearance are to be obtained, use of these conventional brightening agents alone are still insufficent to provide a practical plating bath and process for industrial purposes.
Recently, it has been found that water-soluble polyamine sulfone compounds can be utilized and are useful as a brightening agent for alkali-zincate type zinc plating systems.
For example, U.S. Pat. No. 4,030,987 of Fujita et al discloses a class of such polyamine sulfone compounds which when utilized together with an aromatic aldehyde in an alkali-zincate plating system provide improved brightness and have stability equal or superior to that obtained when plating from a zinc plating bath containing a cyanic compound. However, the combination of such polyamine sulfone compounds with an aromatic aldehyde restricts the operating conditions, particularly with regard to temperature, as a result of the aforementioned fact that aromatic aldehydes are unstable in an alkaline solution and will deteriorate more rapidly at higher temperatures. Thus, the additional desired benefits of operating such a cyanide-free plating system at higher temperatures cannot be readily obtained in accordance with the non-cyanide plating system disclosed by U.S. Pat. No. 4,030,987.